in the location of the hyperbola, which is regarded not as a sharp line but as a 

 band or zone. 



The actual uncertainty in the location of the navigator's line of position 

 is the product of the timing and positional uncertainties . K three -eights of a 

 mile is considered a typical positional uncertainty and 2.5 microseconds a 

 typical timing uncertainty for a hyperbola obtained by ground-wave reception 

 from two Loran stations, then the actual position uncertainty for daytime opera- 

 tion is about 1 mile. K sky wave reception is used and three-fourths of a mile 

 and 8 microseconds are the corresponding quantities, then the actual nighttime 

 uncertainty would be about 6 miles. These figures are estimates only and not 

 averages of a series . This margin of uncertainty is larger than found in practice 

 and can be reduced by a factor of about 2 under favorable conditions . A Loran 

 fix is normally expected to be compared to a good celestial observation. 



Over large parts of the service areas of Loran, hyperbolic line cross- 

 ing angles of 30° or better are obtained which allows a relatively good fix to be 

 obtained. At the extreme limits of sky-wave coverage (approximately 1400 

 miles) crossing angles may be quite small if all stations are situated along one 

 coast or in some similar arrangement. In this situation, positions are obtained 

 with relatively good accuracy in a direction perpendicular to the lines of the 

 position, but with relatively poor accuracy along the lines of position. Where 

 crossing angles are small, the practice of averaging a number of readings should 

 result in improved readings . 



The uncertainty in plotting a position can be reduced by using three or 

 more Loran lines of position or by combining Loran lines with those from another 

 source. Careful consideration should be given to the relative accuracy of various 

 Loran lines . 



4. SUMMARY 



a . Type of System 



(1) Pulsed time -difference hyperbolic plot 



(2) Length of base line: 



(a) average = 200 - 400 miles 



(b) long = 200-700 miles 



65 



arthiir ai.lLittlf.Ilnr. 



